In collaborative studies with the Center for Ulcer Research and Education, V.A, Hospital, Los Angeles, the histamine-containing cell in dog gastric mucosa has been identified as a small mast cell with a histamine content of around 3 pg/cell. Histidine decarboxylase activity is also present in small amounts in this cell. The cell exhibits the characteristic metachromasia upon staining with toluidine blue and like the blood basophil is resistant to the histamine-depleting action of compound 48/80. Fractionation of other mucosal cells by centrifugal elutriation has further shown that the enzyme responsible for histamine degradation, histamine-N-methyltransferase (HNMT), is present in the parietal cell of dog mucosa. Additional studies with the new H2 receptor agonist Dimaprit indicates that this compound has a similar affinity for HNMT and the H2 receptor and is a noncompetitive inhibitor of the enzyme. BIBLIOGRAPHIC REFERENCES: Shaff, R.E. and Beaven, M.A.: Turnover and synthesis of diamine oxidase (DAO) in rat tissues. Studies with heparin and cycloheximide. Biochem. Pharmac. 25: 1057-1062, 1976. Beaven, M.A.: Histamine and its participation in dermatological disorders, Part 1. Progr. Dermatol. 11: 15-22, 1977.